The present invention pertains to a method for continuously removing volatile organic material from an aqueous dispersion of synthetic polymers. In particular, it is directed to a method for continuously removing residual monomers from a latex obtained from an emulsion polymerization reaction.
The production of a synthetic polymer may be conventionally carried out by polymerizing the constituent monomer or monomers thereof using an aqueous emulsion, suspension, or microsuspension process. The resulting product, being in the form of an aqueous latex or slurry, i.e., an aqueous dispersion, invariably contains a significant quantity of residual monomer and other non-monomeric impurities, such as organic solvents and the like. In many instances, it is desirable to remove substantially all of the residual monomer in the dispersion, especially in those cases where the quality of the polymer is thereby improved, where the monomer merits recovery for further use, and/or where the presence of even small quantities of residual monomer in the polymeric product would create problems in handling and use due to toxicological hazards. In cases where the monomer is a low boiling liquid, e.g., vinyl chloride and vinylidene chloride, and the polymerization has been carried out under autogenous pressure, much of the residual monomer remaining in the reaction vessel at the end of polymerization can be removed by simply venting the vessel. Nevertheless, the resulting dispersion will still contain an appreciable amount of residual monomer.
Conventionally, residual monomers have been removed from an aqueous dispersion after emulsion or suspension polymerization by heating and/or reducing the pressure within the reaction vessel. However, this method alone is not acceptable for larger and deeper reaction vessels, as the depth of the dispersion will prevent uniform monomer escape due to the difference in hydrostatic pressure between the top and bottom of the vessel. Accordingly, it has been suggested to agitate or circulate the dispersion within the reaction vessel in order to overcome these difficulties. Even with these improvements, however, monomer removal takes an extended and uneconomical period of time. Furthermore, when some polymers, e.g., polyvinyl chloride and polymers of vinylidene chloride, such as polyvinylidene chloride, are subjected to high temperatures for even short periods of time, the quality of the product is severely reduced.
It is also known to remove residual monomers by contacting an aqueous dispersion with a hot inert gas, such as nitrogen, or vapor, such as steam. The efficiency of such a process may be improved by operating at an elevated temperature and under a reduced pressure. Such a process, however, involves large quantities of gas which causes foaming problems and requires uneconomically long treating periods. Furthermore, the process is generally unacceptable for treating polymer latices, especially when performed as a batch operation, since localized hot spots caused by the gas bubbles, as well as prolonged exposure to high temperatures, will impair the stability of the latex and cause problems with coagulation and degradation of the polymer. It has been suggested that some of these problems could be overcome, by using a continuous process, which would reduce the period during which the latex is in contact with the hot gas. Such a modification does not, however, alleviate the problem of post-treating the increased volume of vapor liberated by such a gas stripping process in order to recover the volatilized organic material. In addition, it has generally been suggested that this continuous process be carried out by passing a dispersion mixed with the hot gas into an evacuated chamber in a finely dispersed form, e.g., in the form of droplets or fine streams. Such a modification, though operable when treating a slurry formed from suspension polymerization, is not acceptable for many polymer latices, since the shearing forces that arise from forming droplets or fine streams would cause coagulation problems.
Accordingly, it would be desirable to have a process for removing volatile organic material from an aqueous dispersion of synthetic polymers which would not require prolonged exposure of the dispersion to elevated temperatures, the addition of steam or other gasses, or subjecting the dispersion to the shearing forces of nozzles and the like. Additionally, it would be desirable to have a process which could be operated at low temperatures without creating excessive foaming problems, and which would remove volatile organic material in a very short time. Specifically, it would be desirable to have a process suitable for removing residual monomers from an aqueous synthetic polymer latex obtained from an emulsion polymerization reaction.